1. Field of the Invention
This invention relates to signal processing for enhancing detection resolution on the basis of a periodic signal obtained in a displacement detecting apparatus such as an encoder or an interferometer.
2. Related Background Art
In an optical incremental type rotary encoder or linear encoder, the displacement or the direction of movement of a moving object is detected by the use of two sine wave signals differing in phase (two-phase sine wave signals, in other words, a sine wave signal and a cosine wave signal) obtained from light receiving means. There is known a technique of producing signals differing in phase from said sine wave signal and said cosine wave signal by a signal interpolating process and increasing the number of divided pulses to thereby enhance detection resolution.
FIG. 1 of the accompanying drawings is a construction diagram showing an example of the signal processing means, and FIG. 2 of the accompanying drawings shows signal waveforms in the various circuit portions of FIG. 1. In FIG. 1, the reference numerals 21 and 22 designate input terminals to which a sine wave signal S1 and a cosine wave signal C1 are input. That is, in contrast with a signal a (sine wave) from the input terminal 21, a signal b (cosine wave) having a phase difference of 90.degree. is input from the input terminal 22. Also, a signal c given a phase difference of 180.degree. with respect to the signal a is obtained in an inverting circuit 23. These three signals a, b and c are suitably weight-added by resistors R, and a sine wave signal of any phase (angle) is interpolated. In FIG. 1, all the resistors R are of the same value and signals d and e of phase differences 45.degree. and 135.degree., respectively, are obtained.
These signals are converted into rectangular waves by four comparators 24, 25, 26 and 27, but usually the inputs of the comparators are given a hysteresis so that the comparators may not cause chattering by the influence of the noise at this time. A pulse circuit 28 constitutes means for obtaining rectangular wave signals of two phases having a phase difference of 90.degree. therebetween. In this manner, signals of higher resolution can be obtained from the sine wave signal and cosine wave signal obtained from the light receiving means.
Also, discretely from this, an attempt has been made to provide an electrical interpolation method using a combination of an analog multiplier and said resistance division as shown in Japanese Laid-Open Patent Application No. 3-99219, or to output interpolation data in a memory with a value which is obtained by A/D-converting two-phase sine wave signals as an address, thereby obtaining high resolution.
However, the above-described technique of interpolating an original signal from the phase information of the two-phase sine wave signals of the displacement detecting apparatus such as the encoder or the laser interferometer and enhancing the resolution has suffered from the following problems.
Generally in the encoder or the interferometer, there is a case where the quantity of reflected light, the quantity of transmitted light or the interference state varies with the movement of an object to be measured, whereby the magnitude of the amplitude of a periodic signal obtained is varied. In such a case, the following inconveniences occur:
(1) When phase division is to be effected by the construction as shown in FIG. 1, if the amplitudes of the input signals S1 and C1 vary, the phase with which the rectangular wave is output will be varied by a hysteresis voltage as indicated by dotted lines in FIG. 2;
(2) When two-phase sine wave signals are to be directly A/D-converted, if the signals become small, the quantization error of the A/D converter will become unnegligible and division accuracy will be reduced; and
(3) When as shown in the aforementioned Japanese Laid-Open Patent Application No. 3-99219, an attempt is made to enhance the resolution by the use of the analog multiplier, the amplitude of the output of the multiplier is proportional to the square of the amplitude of the two-phase sine wave of the original signal and therefore, if for example, the amplitude of the original signal becomes 1/2, the output of the multiplier will become 1/4 and the S/N ratio will be extremely reduced.